I know this feel subjective but from my observation on programing life and daily life, the number 3 appears everywhere.

• [Java, OOP] The best practice says that, the depth of class inheritance should be 2 or 3, but not more than that
• [Javascript, callback] The best practice says that, Callback hell refers to the situation where callbacks are nested within other callbacks several levels deep. From 3 level depth, the source code is potentially difficult to understand.
• [UI UX] Well, that’s a good goal for a UX designer to achieve. The 3-Click Rule states that users should be able to perform a task in 3 clicks.
• [System design] Most of system is 3-layer, or 3-tier design, such as MVC, MVP
• [3D design] 3D models are formed from triangles.
• [Writing] Power phrases are formed from 3 consecutive nouns, verbs, or adjectives
• [Time] The past, the present, the future
• [Religion] Faith and Hope and Charity
• [Mind] The heart, the brain, the body
• [Government] Separation of Powers is the concept whereby power must be divided into 3: the legislative, executive and judicial branches
• [Dating] It usually takes three dates for you to know if this person is good enough for you to keep them in your life
• [Survival] You can survive three minutes without breathable air (unconsciousness), or in icy water. You can survive three hours in a harsh environment (extreme heat or cold). You can survive three days without drinkable water. You can survive three weeks without food.
• [Lucky] “Third time lucky”.
• [Competition] A stable competitive market never has more than three significant competitors.
• [Photograph] A composition guideline: places your subject in the left or right third of an image, leaving the other two thirds more open. This create compelling photos.
• [Music] The idea is to have only three musical phrases playing at any one time in your song. Going beyond three or four elements at once can crowd your track, making it harder for your audience to connect and recall your composition.
• [Music] Each chord is formed from 3 notes.
• [Music] Basic rock/pop song structure generally has three unique parts: The verse, the chorus, and the bridge.
• [Golden Circle] Why, How, What
• [Teaching/Learning] Give students the opportunity to learn something at least three times before they are expected to know it and apply it.

And, there is a rule with name :”Rule of Three”.